Alum containing antioxidant and manufacture of sized paper therewith



ALUM CONTAINING ANTIOXIDANT AND MANU- FACTURE OF SIZED PAPER THEREWITH David Lnrie, Freeport, N. Y.,' assignor to American Cyanaimid Company, New York, N. Y., a corporation of Ma ne No Drawing. Application Api'il 28, 1955 Serial N0. 504,685

Claims. (Cl. 92-21) webs thus produced, and further includes dry alum andaqueous alum solutions containing a small amount of a suitable thiosulfate. f

The manufacture of soap-sized cellulosic webs for many years has been and still is a principal product of the paper industry. Such webs are manufactured by forming an aqueous suspension of beaten papermaking fibers, adding rosin size, fortified rosin size, stearic acid size or other soap size, and precipitating the size on the fibers by addition of a water-soluble aluminum salt.

Rosin size and alum (papermakers alum) are the two materials employed in greatest tonnages for this purpose, and the present invention is most conveniently discussed in terms thereof, it being understood that neither the type nor amount of size used is a particular feature of the invention. The invention, however, is directly concerned with the alum employed.

Alum is usually supplied to paper manufacturers in two grades. The standard grade is produced by direct digestion of bauxite or clay with sulfuric acid, and thus contains a minor amount of iron as contaminant, roughly 0.3% to about 0.5% by weight calculated as FeiOa based on the weight of the alum calculated as aluminum sulfate tetradecylhydrate. All of this iron necessarily is in sulfate form, and is usually a ferric-ferrous mixture. The iron impurities have about the same elfectiveness as alum for precipitating size, and thus are carried to the fibers both in ferrous and ferric form.

It is known that rosin-sized paper made by the use of standard" grade alum on exposure to sunlight loses reflectance or darkens. The data indicate that this darkening results from decomposition of ferrous rosinate into ferric hydroxide, a water-insoluble, veryopaque, and dark brown material which is thus an excellent pigment. Only a trace is sufiicient to cause a reflectance loss of 10%- and often more.

Although this darkening has been often noted, no simple, cheap and safe means has as'yet been found for preventing or even inhibiting the phenomenon.

I have now discovered that this darkening can be retarded or inhibited by precipitating the size on the fibersin the presence of a small amount of a dissolved ionized thiosulfate. A principal resulting effect is the adsorption of thiosulfate by the size, the fibers, or any aluminum hydroxide 'floc present whereby thiosulfateiscztrried to the fibers along with the ferrous rosinate which is also formed and maintains the ferrous rosinate in reduced, colorless condition whereby darkening of the paper resulting from oxidation of this iron is inhibited.

Patented Feb. 24, 1959 It is a particular advantage of the present invention that addition of a thiosulfate as described causes no noticeable change in the character of the paper produced, other than to improve and maintain its brightness.

The water-soluble ionizable thiosulfates have the property of reducing dissolved ferric iron to ferrous iron almost instantly. It will be realized, therefore, that the process of the present invention by requiring that the sizing agents be deposited on the cellulosic fibers in the presence of dissolved thiosulfate inherently requires that the slurry at that point be free from dissolved ferric iron.

The precise form or forms in which the thiosulfate is present in the web has not been ascertained, and thus it is not known whether the thiosulfate is molecularly adsorbed, or whether only the thiosulfate anion is present in the paper. in terms of itsprocess of manufacture.

Methods for employing the present invention need not be discussed in detail as it may be put into use simply as one additional step in any conventional process for the manufacture of soap-sized paper. All that is necessary is to introduce a suitable thiosulfate (alone or in admixture with other material) into the pulp at such a point in the operation that a small amount of thiosulfate is present in uniformly distributed, dissolved form while the size is being set on the fibers. For example, inmills where the size is added first the thiosulfate may be added before, with or after the size so long as it is added before the alum. In mills which add the rosin last, thethiosulfate may be added before, with or after the alum so long asit is added before the rosin.

The thiosulfate may be added as a dry powder. Better results generally ensue, however, when it is added as a solution. Wet and dry-strength resins may be added in the usual manner after the size has set.

Ferrous sulfate in aqueous solution is readily oxidized to ferric sulfate by simple aeration of the solution. To minimize consumption of thiosulfate it is therefore advantageous to add the alum and thiosulfate as close to the machine as convenient.

The foregoing methods contemplate that the minimum effective amount of thiosulfate be determined by the paper manufacturer. It is often more convenient for this'to be determined by the alum manufacturer. This is possible when the alum is shipped in dry form. We have found that at most only negligible deterioration of thiosulfate thiosulfate is predetermined by the alum manufacture, so

that when the blend is dissolved in water (1) any ferric iron present will be reduced to ferrous iron and (2) the solution will then contain a small but effective amount of thiosulfate as anti-oxidant based on the weight of the alum. Generally the solution should contain between about 0.5% and 4% of thiosulfate, based on the dry Weight of the alum, depending on the amount of antioxidant action desired.

The amount of thiosulfate required to reduce the ferric iron can be accurately determined by known methods of analysis. The amount of thiosulfate required for this step depends of course on the precise amount of trivalent iron present. Since this varies from instance to instance, the amount cannot be stated numerically.

The amount of excess thiosulfate should be sufficient to provide efiective anti-oxidant action for the ferrous iron present. A distinct inhibition of oxidation has been observed when fibers have been rosin-sized in the presence of only 0.1% of thiosulfate based on the weight of the alum. Evidently then there is no amount, however,

The web is thus most conveniently defined 3 small which does not exert at least some antioxidant action.

l%5% of sodium thiosulfate based on the weight of the alum. Thiosulfate within this range is suflicient to reduce all the ferric iron present in commercial alum even after it has been extensively aerated and provide a small but adequate residuum of thiosulfate as oxidation inhibitor for the ferrous iron. Less is added in the case of short-lived paper such as carton stock, and more in the case of permanent paper such as book paper.

Fixation of thiosulfate on the fibers is favored by addition of a small amount of sodium aluminate or sodium phosphate aluminate as auxiliary adsorbent, for example about 0.25 to 0.75% based on the dry weight of the fibers. These materials hydrolyze to form aluminum hydrate floc which provides additional surface on which thiosulfate can be adsorbed or occludedand carried to the fibers, resulting in better thiosulfate retention.

Although only sodium thiosulfate has been tried, no reason is seen why all water-soluble ionizable thiosulfates should not also prove useful, as their principal function is only to supply thiosulfate ions. All the water-soluble ionizable thiosulfates are therefore included within the scope. of the invention and the calcium, sodium, ammonium, potassium, and tetramethyl ammonium thiosul fates are particularly mentioned in this regard since they are either commercially available or can readily become so.

By soap-sizedcellulosic webs there is meant ordinary paper or other web manufactured by a process which includes the steps of adding between about 0.2% and 3.5% of a soap size (soap solids based on the dry weight of the fibers) to an aqueous slurry of cellulosic fibers, setting the size on the fibers by alum in amount between about 100% and 150% of the size, sheeting the fibers on a. wire screen, and drying the web thus formed. The web may contain any of the customary starches, gums, Waxes, and wetand dry-strength resins employed in the manufacture of cellulosic webs. The thiosulfates of the present invention are very mild reducing agents and have no significant effect thereon.

In the specification and claims weights are calculated on the solids in the size, on alum calculated as aluminum sulfate tetradecyl hydrate, and on thiosulfate as sodium thiosulfate pentahydrate, as the case may be. It will be understood that the particular sizes employed, the amount of water of hydration in the alum or thiosulfate, and the manner in which the webs are manufactured are not principal features of the invention.

The invention will be further illustrated by the examples which follow. These examples are specific embodiments of the invention and are not to be construed as limitations thereon.

Example I 196% 'of which is ferrous sulfate, the remainder being --ferric sulfate.

The-resultingthiosulfate-alum blend was used in the manufacture of -3% rosin-sized light-weight carton board as a replacement for the same alum which contained no thiosulfate.

Samples .of the board made with and withoutthiosulfate were exposed to the sun for several weeks. During the test the board containing thiosulfate remained notice- In practice good results have been obtained by adding ably brighter than the board which contained no thiosulfate.

Example 2 The following illustrates the manufacture of commercial liquid alum containing thiosulfate according to the present invention.

A large digestion batch of liquid alum was analyzed and was found to contain 49.0% by weight of alum as Al (SO .14H O, 7.1% insolubles, and 0.6% of ferrous iron and 0.9% of ferric iron both calculated as Fe. The liquid alum had been prepared by digestion of bauxite with hot H 80 to form an alum solution containing a 0.5% A1 0 excess-(dry basis) and adding sufiicient wash water to dilute to the equivalent of 36' B6. at F.

Calculations showed that 12.5 lbs. of sodium thiosulfate pentahydrate are needed per 1000 gallons of the liquid alum to reduce all the iron to divalent form. Moreover, it was known from experience that an additional 95.5 lbs. of the thiosulfate should be added per 1000 gallons of alum for antioxidant purposes.

The alum solution was pumped into a settling tank, and during the pumping 108 lbs. of sodium thiosulfate was added to the settling tank where complete mixing took place. When the insolubles were less than 0.05%, the alum was pumped to storage and used for the manufacture of rosin-sized paper.

Example 3 The effect of the presence of thiosulfate in paper was determined in the laboratory by. comparative experiments as follows.

Three sets of rosin-sized handsheets were prepared which were identical except for the particular alum employed for setting the size.

Each set of handsheets was prepared by adding 3% of gum rosin size and 3% of one of the alums described below (based on the dry weight of the fibers) to a 95-g. batch of bleached sulfitepulp at 2.5% consistency. The pulp had been beaten to a Green freeness of 350 ml. The pulp was stirred for five minutes after each addition and then diluted to 0.6% consistency. Two 4.8-g. British sheets were made using fresh deionized water for each. This water was returned to the sump for the white water system. Four 4.8-g. sheets were then made to build up the white water system. More alum (11 ml. of a 10% solution) was added at the sump to the white water systern. The pH of the system was 4.5 which was the H +O concentration throughout each run. Eighteen 3-g. sheets were then made, which were pressed on a British press between blotters and dried on a Noble and Wood drum drier at 240 F.

Three alums were used as follows.

Alum A was an iron-free alum containing (dry basis) 20 p. p. m. of iron.

Alum B was a commercial alum containing a normal amount of iron. Alum C was alum B to. which 2.0% of sodium thiosulfate pentahydrate had been added based on the weight of the AI (SO .14H O therein.

The analysis of the three samples of alum, as 10% aqueous solution by volume, were as follows.

Iron (as F0203) Alum Al as Ferrous Ferric Total 1 16. 2 l6. 4 0. 31 0. 15 0.46 16. 3 0. 46 Nil 0 40 1 MgJml. of solution. I

The reflectance of the samples at 450 mg was determined against a standard MgCO block. The samples were then exposed to artificial sunlight for 16 hours in'a well ventilated accelerated tested of the Fade-O-meter type. The samples were mounted radially 10 inches from the carbon arc. The samples were then removed and their reflectance determined in the same manner. The values obtained were averaged and are as follows.

The sheets are identified by the alum used their prepa- The results on the sheets containing standard alum (sheets B and C) show that the presence of thiosulfate decreased the amount of darkening by nearly one-half (41%). Comparison of sheet C with sheet A shows that the addition of thiosulfate gave results which were quite comparable to the results obtained with iron-free alum.

I claim: 1. In the manufacture of a sized cellulosic web wherein an aqueous suspension of cellulosic fibers is formed, a soap size is precipitated on the'fibers by the action of alum containing a minor amount of iron sulfate as impurity, and the thus sized fibers are sheeted to form a web having a content of iron: the method of inhibiting the darkening which said web undergoes on aging due to oxidation of ferrous iron therein which con- 1 sists essentially in precipitating the size on the fibers 1n the presence of a ionizable thiosulfate dissolved m the aqueous phase of said suspension and thereby carrying a small but effective amount of the thiosulfate to said fibers as antioxidant for said iron. 7

2. A process according to claim 1 wherein the soap size is'a rosin size.

3. A process according to claim 1, wherein therthiosulfate is present in the fibrous suspension prior toprecipitation of the size.

4. A process according to claim 1 wherein the thiosulfate is sodium thiosulfate.

5. A cellulosic web corresponding to that produced by the process of claim 1.

6. In the manufacture of a sized cellulosic web,

v wherein an aqueous suspension of cellulosic fibers is formed, a soap size is precipitated on the fibers by the action of an alum containinga minor amount of ferrous sulfate as impurity, and the thus sized fibers are sheeted to form a web having a content of ferrous iron: the

method of inhibiting the discoloration which said web 10 undergoes on aging as a result of oxidation of the ferrous iron therein which consists essentially in premixing with said alumsufficient of a water-soluble ionizable thiosulfate to reduce any ferric sulfate therein to ferrous sulfate and to supply a small but effective amount of thiosulfate as anti-oxidant for the ferrous iron to be deposited in said web.

7. A composition for use in paper making comprising papermakers alum and a water-soluble ionizable thiosulfate, said alum containing a minor amount of at least one iron sulfate as impurity, the amount of said thim sulfate being sufficient '(l) to reduce any trivalent iron 1 present to ferrous iron and (2) to provide a small but elfective amount of said thiosulfate as antioxidant for said ferrous iron.

8. A blend according to claim 7 wherein thiosulfate is sodium thiosulfate.

9. A composition according to claim 7, wherein the blend is a liquid solution.

10. A composition according to claim 7, wherein the blend is dry.

References Cited in the file of this patent UNITED STATES PATENTS 2,154,996 Rawling Apr. 18, 1939 2,199,829 Bogdan May 7, 1940 1 2,318,898 Stiegler May 11, 1 943 OTHER REFERENCES Casey: Pulp and Paper, vol. I, Interscience Publishers,

N. Y., 1952, pages 516 and 517.

'Mellor: Modern Inorganic Chemistry, pub. by Longmans, Green and Co., N. Y., 1925, page 548. Pulp and Paper Manufacture, vol. 2, McGraw-Hill Book Co., New York (1951), page 177. 

1. A COMPOSITION FOR USE IN PAPER MAKING COMPRISING PAPERMAKER''S ALUM AND A WATER-SOLUBLE IONIZABLE THIOSULFATE, SAID CONTAINING A MINOR AMOUNT OF AT LEAST ONE IRON SULFATE AS IMPURITY, THE AMOUNT OF SAID THIOSULFATE BEING SUFFICIENT (1) TO REDUCE ANY TRIVALENT IRON PRESENT TO FERROUS IRON AND (2) TO PROVIDE A SMALL BUT EFFECTIVE AMOUNT OF SAID THISULFATE AS ANTIOXIDANT FOR SAID FERROUS IRON. 